Rotary printing presses conventionally include ink fountains with zone width adjustments across the fountain for controlling ink feed in strip-like regions positioned side-by-side across the printed form. The adjusters can take various mechanical forms, but as a common feature, they typically control the ink film thickness in respective zonal widths across the fountain, and the ink film thickness as applied to the form controls the printed optical density of the form. In multicolor printing work, it is typical to individually control the film thicknesses of the individual colors which are superimposed, and the quality of the final printed result is determined by the control exercised over the film thicknesses of the individual ink films.
Printing plants capable of producing the highest quality product have evolved toward a high standard of automation wherein image scanners are utilized to scan the printed image and have automatic control systems coupled to the scanner for directly controlling ink adjustment on the press. Without describing all of the complex interrelationships necessary for successful operation of such a high quality feedback system, it will be appreciated that at a minimum there is required a scanner (such as a densitometer) which is automatically positioned by the control system with respect to the form to provide position feedback for the scanner to the control system. In order for the control system to accomplish its function of adjusting the appropriate ink keys to produce the desired printed density on the printed form, the control system must know the zone being scanned by the scanner at the time the reading is made. As a result, such automatic control systems often utilize relatively complex scanning mechanisms with scanning heads which are automatically positioned by motor controlled closed loop control systems to read the image while at the same time indicating position of the scanner head with respect to the printed form. As an alternative, scanning elements are sometimes positioned across the press in fixed locations, and their signal is known to relate to ink film thickness produced by one or more ink feed mechanisms located in a known positional relationship with respect to the scanner.
Many fully automated scanning and adjustment systems have also evolved to require an ink check strip printed along one of the borders of the printed form. The scanner scans the check strip rather than the image in order to eliminate as many variables as possible which might result from scanning the image. The check strip usually includes both solids and screens of the individual colors which have produced the overall printed image, and scanning of those individual patches in the check strip provides data which is processed to determine the nature of the ink adjustment to be applied to each zone of the press.
Such automatic control systems can be relatively expensive and can require relatively sophisticated operators to exercise the necessary care and skill in seeing to the appropriate adjustments. As a result, much printing today is still produced in printing plants where quality control on the press relies largely on the subjective judgment of the pressman viewing samples of the printed form. Occasionally, an off-line densitometer is available to quantify the subjective judgment at particular key points in the image, but in the end it is the subjective judgment of the operator, sometimes aided by densitometric quantitative data, which results in the operator selecting one or more ink keys for adjustment to alter the image quality of the printed form.